GUI: Changed the script alterTendonSlackLength.py to handle when no model is loaded in the GUI
GUI: Changed the script alterTendonSlackLength.py to handle when no model is loaded in the GUI

git-svn-id: 9e047e1e624cce0a2485e750063112e834f85522@8203 fc29be9c-2cb0-420d-b520-73dbd18add6e

opensim-org/opensim-gui,opensim-org/opensim-gui,opensim-org/opensim-gui,opensim-org/opensim-gui